Factor out the build_ext fixup for shared Python builds.

I need this to fix the failing test_command_install_dist.

Lib/packaging/tests/support.py

@@ -28,6 +28,7 @@ Each class or function has a docstring to explain its purpose and usage.
 """
 
 import os
+import sys
 import shutil
 import logging
 import weakref
@@ -279,7 +280,7 @@ def copy_xxmodule_c(directory):
 
         def test_compile(self):
             copy_xxmodule_c(self.tmpdir)
-            self.assertIn('xxmodule.c', os.listdir(self.tmpdir)
+            self.assertIn('xxmodule.c', os.listdir(self.tmpdir))
 
     If the source file can be found, it will be copied to *directory*.  If not,
     the test will be skipped.  Errors during copy are not caught.
@@ -304,6 +305,31 @@ def _get_xxmodule_path():
             return path
 
 
+def fixup_build_ext(cmd):
+    """Function needed to make build_ext tests pass on shared builds.
+
+    When Python was build with --enable-shared, -L. is not good enough to find
+    the libpython<blah>.so.  This is because regrtest runs it under a tempdir,
+    not in the top level where the .so lives.  By the time we've gotten here,
+    Python's already been chdir'd to the tempdir.  This function work arounds
+    that.  Example use:
+
+        cmd = build_ext(dist)
+        support.fixup_build_ext(cmd)
+        cmd.ensure_finalized()
+    """
+    # To further add to the fun, we can't just add library_dirs to the
+    # Extension() instance because that doesn't get plumbed through to the
+    # final compiler command.
+    if (sysconfig.get_config_var('Py_ENABLE_SHARED') and
+        not sys.platform.startswith('win')):
+        runshared = sysconfig.get_config_var('RUNSHARED')
+        if runshared is None:
+            cmd.library_dirs = ['.']
+        else:
+            name, equals, value = runshared.partition('=')
+            cmd.library_dirs = value.split(os.pathsep)
+
 try:
     from test.support import skip_unless_symlink
 except ImportError:

Lib/packaging/tests/test_command_build_ext.py

@@ -32,25 +32,6 @@ class BuildExtTestCase(support.TempdirManager,
 
         super(BuildExtTestCase, self).tearDown()
 
-    def _fixup_command(self, cmd):
-        # When Python was build with --enable-shared, -L. is not good enough
-        # to find the libpython<blah>.so.  This is because regrtest runs it
-        # under a tempdir, not in the top level where the .so lives.  By the
-        # time we've gotten here, Python's already been chdir'd to the
-        # tempdir.
-        #
-        # To further add to the fun, we can't just add library_dirs to the
-        # Extension() instance because that doesn't get plumbed through to the
-        # final compiler command.
-        if (sysconfig.get_config_var('Py_ENABLE_SHARED') and
-            not sys.platform.startswith('win')):
-            runshared = sysconfig.get_config_var('RUNSHARED')
-            if runshared is None:
-                cmd.library_dirs = ['.']
-            else:
-                name, equals, value = runshared.partition('=')
-                cmd.library_dirs = value.split(os.pathsep)
-
     def test_build_ext(self):
         support.copy_xxmodule_c(self.tmp_dir)
         xx_c = os.path.join(self.tmp_dir, 'xxmodule.c')
@@ -58,7 +39,7 @@ class BuildExtTestCase(support.TempdirManager,
         dist = Distribution({'name': 'xx', 'ext_modules': [xx_ext]})
         dist.package_dir = self.tmp_dir
         cmd = build_ext(dist)
-        self._fixup_command(cmd)
+        support.fixup_build_ext(cmd)
 
         if os.name == "nt":
             # On Windows, we must build a debug version iff running
@@ -251,7 +232,7 @@ class BuildExtTestCase(support.TempdirManager,
         dist = Distribution({'name': 'xx',
                              'ext_modules': [ext]})
         cmd = build_ext(dist)
-        self._fixup_command(cmd)
+        support.fixup_build_ext(cmd)
         cmd.ensure_finalized()
         self.assertEqual(len(cmd.get_outputs()), 1)